1. Field of the Invention
The present invention relates to an optical element having fog prevention and antireflection characteristic. In particular, the present invention relates to an optical element such as a photographic lens, a projection lens, a filter, and a mirror which is excellent in fog prevention, antireflection, and weather resistance, or optical equipment such as electrophotographic equipment incorporating such an optical element.
2. Related Background Art
Conventionally, for preventing fog of a lens, a filter, a mirror, and the like, a method of coating a surface with a surfactant is generally conducted. Recently, it is also known that fog is prevented by coating a base such as a lens, a filter, a mirror, or the like with a water-absorbing material instead of a surfactant. Furthermore, conventionally, as a water-absorbing material, known as natural polymers are: starch-based polymers such as a starch-acrylonitrile graft polymer hydrolysate; and cellulose-based polymers such as cellulose-acrylonitrile graft polymer. Known as synthetic polymers are: polyvinyl alcohol-based polymers such as a polyvinyl alcohol cross-linked polymer; acrylic polymers such as a sodium polyacrylate cross-linked substance; and polyether-based polymers such as a polyethylene glycol/diacrylate cross-linked polymer, etc.
However, the above-mentioned conventional fog prevention optical elements have the following problems. First, in the case where a surfactant is used for preventing fog, duration of its effect is very short, and unless the surfactant is applied again within several hours or days, its effect cannot be maintained. Furthermore, in the case where dirt on the surface of an optical element is wiped off with water or the like, a surfactant film is removed, and its effect is lost.
Furthermore, in the case where various water-absorbing materials are applied to form a fog prevention film for preventing fog, the duration of its effect is remarkably enhanced as compared with the case of using a surfactant. However, according to the study by the inventors of the present invention, the following was found: in the case where the water-absorbing material is used for a fog prevention film, when a low refractive material layer is coated onto the fog prevention film so as to obtain an antireflection effect, the fog prevention characteristic tends to be lost. Furthermore, in the case where the water absorption layer is made thin, and the thickness of an optical film is adjusted to an odd multiple of one-quarter of the wavelength of antireflection target to obtain an antireflection film, the thickness of the water-absorbing film becomes too thin, and sufficient fog prevention characteristic cannot be obtained.
In order to solve the above-mentioned problems, the inventors of the present invention have proposed, in Japanese Patent Application Laid-Open No. 11-109105, of forming films with different refractive indexes on a water-absorbing film to obtain an antireflection film. However, in this composition, there is no problem in the case where a lens and a filter with a refractive index nd of about 1.50 are used; however, in the case where a glass base having a high refractive index (nd>1.60) is used, interference fringes become conspicuous between the water absorption film and the base due to even a slight difference in thickness, so that a defect is observed regarding its outer appearance.